As the above type of automated storage/retrieval system, for example, the automated storage/retrieval system described in Japanese Patent Application Publication No. JPH08-324721 is well-known. The automated storage/retrieval system described in JPH08-324721 includes at least one pair of left and right multi-tier racks, each being composed of multi-tier shelves. Between these multi-tier racks are provided a transferring shuttle that can run in a horizontal direction. The transferring shuttle is for storing a load to or retrieving a load from the left or right multi-tier racks.
The automated storage/retrieval system described in such Japanese patent application publication is referred to as a captive-type automated storage/retrieval system since one transferring shuttle is exclusively provided for each tier of the multi-tier racks. This captive-type automated storage/retrieval system is highly efficient, but the increase of the number of tiers of the multi-tier racks increases the number of expensive transferring shuttles and the number of waiting transfer shuttles thereof, which in turn increases the whole cost of the automated storage/retrieval system.
Then, conventionally, as described in Japanese Patent Application Publication No. 11-278607, an automated storage/retrieval system is known in which an elevator to move a transferring shuttle upward and downward is disposed adjacent to a multi-tier rack. That is, by using the elevator to move the transferring shuttle to another tier, the number of transferring shuttles can be made to be less than the number of tiers of the multi-tier racks. Such an automated storage/retrieval system is referred to as a roaming-type automated storage/retrieval system since the transferring shuttle can move to any tier of the multi-tier racks.